This invention relates to gas flow measuring devices of high accuracy, preferably with an accuracy of below 0.5% absolute, for use as a cost-effective primary (dimensionally traceable) flow calibration device and for calibration of other gas flow measuring devices particularly for field service application as well as for use with mass flow meters and mass flow controllers.
There is a need in the industry for a precise gas flow measuring device which can accurately measure gas flow in the 1 sccm to 50,000 sccm gas flow range. The device should possess a high turndown ratio of more than 100 to one with respect to the speed of operation so that only one gas flow measuring device is needed to cover a wide range of gas flow measurements particularly for use in the field service calibration of other gas flow measuring devices. Constant-displacement systems are, perhaps, the simplest and most intuitive flow measurement devices in that they are characterized by the most basic of quantities: length and time with flow mathematically derived therefrom. A dimensionally characterized system would be as close as possible to direct traceability from national dimensional standards.
An idealized constant displacement piston flowmeter would consist of a massless, frictionless, leak proof, shape-invariant and impermeable piston inserted within the flow stream to be measured and enclosed by a perfect cylinder. In an ideal device the time that the piston takes to move a known distance between two points on the cylinder (which implies a known volume) yields the volumetric flow calculated as follows:
F=V/T=xcfx80r2h/T
Where V=displaced volume
T=elapsed time.
FIG. 1 schematically illustrates a basic automatic piston prover. Gas ordinarily passes through the bypass valve. When a reading is called for, the valve closes. The gas is then forced to pass through the cylinder, effectively inserting the piston into the flow stream. After a suitable acceleration interval. A measurement is made of the time required to pass from a first sensor to a second sensor and utilized in the foregoing equation.
There are two constant displacement flowmeters in common use; bell provers and piston provers. National laboratory level primary standard provers are large, expensive, slow and relatively non-automated calibrators Bell provers operate in a manner similar to piston provers, but the enclosed volume consists of an inverted cup-shaped bell with edges submerged in a sealing liquid, such that vertical movement of the bell changes the volume enclosed above the liquid.
A piston prover, in its most basic form, can be nothing more than a calibrated burette within which a soap-film bubble rises in response to gas flow. A stopwatch can be used to time the bubble""s passage through a known volume between two marks. More modern bubble calibrators use optical bubble detection and an internal computer. The accuracy of any practical bubble device is limited by:
Vapor pressure of water
Shape variation of the bubble
Permeability of the bubble
Fluid viscosity changes with evaporation
Variation of cylinder working diameter from dried and prior-reading bubble solution
The above-described uncertainties limit the usefulness of bubble devices. Vapor pressure alone can account for an inaccuracy of xc2x11.5% uncertainty. Bubble devices are of value when the insertion pressure must be as constant as possible, such as measurement of a very highly unregulated source.
All other bell and piston provers, although more accurate than bubble devices (on the order of 0.2%), possess significant insertion pressure. Moreover, the dynamic pressure has recently been investigated as a significant source of measurement uncertainty. Further improvement of prover accuracy, as well as achievement of conventional accuracies in smaller, faster or less expensive provers can be achieved by reducing the effect of dynamic pressure variations. Although this methodology is applicable in a similar manner to all constant-volume provers, we will illustrate its application to viscous-sealed piston provers.
Our preferred embodiment consists of a viscous-sealed piston prover. Viscous-sealed piston provers use a piston and cylinder fitted so closely that the viscosity of the gas under test results in a leakage small enough to be insignificant. For reasonable leakage rates, such a gap must be on the order of 5 to 10 microns. A portable dry piston prover permitting high piston velocities and small measurement distances is taught in U.S. Pat. No. 5,440,925 and U.S. Pat. No. 5,456,107 respectively with each disclosure being herewith incorporated by reference. In the device taught in the foregoing patents the piston and cylinder are respectively made of graphite and borosilicate glass because of their low, matched temperature coefficients of expansion and low friction and, as such, are essentially shape invariant, impermeable and virtually frictionless. There is no vapor pressure from a bubble or sealant. Although the instrument can utilize high piston speeds, resulting in a measurement repetition rate rapid enough to be considered quasi-continuous, the accuracy has heretofore been limited to a maximum of about 0.5%. The best provers have exhibited accuracies on the order of 0.2%.
The purpose of this invention is to enhance the maximum accuracy of constant-volume provers, or to allow smaller size, faster speed or lower cost for a chosen accuracy level. There is no teaching in the prior art or in the aforementioned patents of the existence of dynamic errors or how to correct for such errors.
Dynamic errors may result from dynamic pressure changes during a measuring cycle and, as such, substantially affect the accuracy of the gas flow measurement. By correcting for errors in dynamic pressure the accuracy of the device is enhanced and preferably before the device is to be standardized for statistical accuracy. The standardization procedure is itself conventional and accordingly is not elaborated upon in the subject application.
It has been discovered in accordance with the present invention that the accuracy of a positive displacement piston flowmeter, such as is illustrated in U.S. Pat. No. 5,440,925, can be substantially enhanced permitting an accuracy of below about 0.5% and preferably below 0.2% absolute to be readily achieved. A high precision statistical accuracy is achieved in accordance with the present invention by automatically correcting for any dynamic errors occurring in response to dynamic pressure changes during the gas flow measurement period. In fact, it has been discovered in accordance with the present invention that an error resulting from a dynamic pressure change is critical to assure measurement integrity of the instrument for use as a precision calibrator.
In accordance with the present invention the presence of an error in response to a dynamic pressure change is determined by measuring the dynamic pressure during the timing interval over which the gas flow is measured, preferably at the start and end of the timing cycle, and computing the dynamic pressure error, if any, as follows:   Error  =                    (                              P            2                    -                      P            1                          )                    P        1              ·                  Vm        +        Vd            Vm      
Where P2 and P1 are the pressures at the end and the start of the timed interval, Vm is the volume over-which the flow is timed and Vd is the dead volume (the total contained inventory volume between the flow source and the measured portion of the cylinder). Once an error is determined to exist the computed value of the error may then be directly applied and substracted from the gas flow calculation to establish a dynamically self corrected gas flow having a highly precise accuracy for enabling the device to be used as a calibrator and to calibrate other less accurate gas flow measuring devices.